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Depth finder meaning

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  • Depth finder meaning

    I've owned seadoo's for years, but this year I traded them in for a 2005 Cobalt 220. Since I'm essentially new to boating and don't want to ruin my new boat I was wondering if somebody could help me with the meaning of the numbers I'm getting on my depth finder. If the gauge is reading 3.5' and I have a 36" draft...shouldn't I be hitting the bottom of the launch ramp? (the outdirve is trimmed up.) I think the sensor is located at the bottom of the transom??? With the prop in the water and coming up to the trailer, the gauge usually reads 3.5'. Can anyone estimate how much clearence I have? I guess I'm asking if anyone has any experience with the meaning of the depth gauge reading. In other words, how much clearence do I have when a depth number comes up.

    Secondly what number to should I set low depth alarm for? I think I have it set for 4.5 feet...which is probably something like 6.5 feet...I just don't know. I like the notification alarm that I'm getting shallow, but I would love to shut the alarm off while still getting readings. Since I'm new, I have enough stress at the docks that I don't need an alarm continuing to go off, but would like the continued data. Any help for a newbie would be fantastic! Thanks!

  • #2
    Re: Depth finder meaning

    Hello Jackmowers, and aboard...

    The depth shown on the depth finder is what's under the transducer, so you need to find the transducer and see where that is in relation to the rest of the hull. If it's mounted on the transom, then there is likely to be about a foot of hull, give or take, below the transducer... If it's mounted in a wet-box or through-hull then the depth shown would be below the hull....

    Also, I think your post would do better in the 'Electronics' section rather than here, so I'll just move it across for you....

    Cheers,

    Chris.........
    Last edited by achris; September 25th, 2013, 09:34 PM.
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    The world takes on a whole new perspective when viewed from 100’ below.
    1972 Bertram ‘Bahia Mar’ 20
    2006 Mercruiser 4.3MPI (0W617679) w/Alpha One Gen II (0W829301)
    (Original - 1972 '165' In-line 6. Previous - 1994 4.3LX)


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    • #3
      Re: Depth finder meaning

      I suspect that the 36" draft is with the drive down.
      And as achris says, it depends where the transducer is mounted.
      1980 Sylvan Aluminum 18' Sportster I/O - Mercruiser 140, Honda 7.5 Kicker, Bennett M120 Tabs
      1974 Glastron V-179, Mercruiser 140
      1984 16.5ft. Alumarine Sealark side console with 82 Merc 50
      Manitoba, Canada

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      • #4
        Re: Depth finder meaning

        It is not a absolute reading of the depth. It could be as much as a foot off one way or the other. 3.5 (41 inches) is 5 inches to spare with drives down. Drives up would be somewhere around 27 inches
        Assault Squadron NINE
        River Assault Division NINETY-ONE-1966

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        • #5
          Re: Depth finder meaning

          If the transducer is right at the keel you have at least 3 feet UNDER the Boat.
          The Common Merc Alpha Outdrive descends about 18 inches below the Keel leaving you at least a foot to spare.
          This all assumes flat water. Waves and wakes will make your margin less.
          When idling in skinny water, Trim full up (Not full Trailer UP) and you will get another 4 inches.

          I like to know when I have less than 6 feet under the keel.
          Remember the depth gauge tells the depth of the water under the transom not what's about to come under the bow.
          Bill, Lorain Ohio

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          • #6
            Re: Depth finder meaning

            It also depends on how the unit was adjusted, typically you can set the depth reading to either read from the surface, or the bottom of the boat. There should be an adjustable offset option in the menu

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            • #7
              Re: Depth finder meaning

              Originally posted by Thalasso View Post
              It is not a absolute reading of the depth. It could be as much as a foot off one way or the other.
              The speed of sound thru water is pretty stable and reliable. I'd be interested in how or where you came up with these figures?
              ....

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              • #8
                Re: Depth finder meaning

                Originally posted by dingbat View Post
                The speed of sound thru water is pretty stable and reliable. I'd be interested in how or where you came up with these figures?
                Defining just where the bottom is, when the material is light mucky sediment, is like trying to measure the size of a cotton ball to the hundredth of an inch. The surface is just not very well defined.

                The speed of sound in water changes more than 5% between 40 and 80 degree water.
                Add in the effect of varying suspended solids (Sediment) and dissolved minerals (Salinity) and the reported depth can vary as much as a foot for every 15 feet of depth.

                Waves and wakes add to the issue.
                As does the calibration of the individual unit and the accuracy and stability of its internal time base.

                The sounder may give you a warm fuzzy feeling when it displays 3.7 feet. But it really means ABOUT 3.7 feet.
                Bill, Lorain Ohio

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                • #9
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                  Re: Depth finder meaning

                  Defining just where the bottom is, when the material is light mucky sediment, is like trying to measure the size of a cotton ball to the hundredth of an inch. The surface is just not very well defined.
                  Funny you should say that. I design, build and sale instruments used to measure things that are much smaller and difficult to measure than your cotton ball………w/o touching them with an accuracy +/- 0.1%. All it takes is an understanding of the physics and a methodology to address the variables. Measuring the depth of the water would be a cake walk.

                  The effects of the variables you mention are well understood. Things like salinity are easily addressed with “soft” settings. The effect of suspend solids on sonar’s response is well understood. A good bottom detection algorithm will negate or minimize its effect. The remaining variables are addressed thru normalization and averaging to keep errors to a minimum.


                  There is no such thing as an absolute measurement, but a good sounder will get you a lot closer than you think.
                  Last edited by dingbat; September 27th, 2013, 03:16 PM.
                  ....

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